The present invention relates to methods for making equipment for diagnostic and therapeutic radiology and, more particularly, to methods for making high performance target/stem assemblies used in x-ray generating equipment, such as computerized axial tomography (C.A.T.) scanners. More particularly, the invention is directed to methods for making high performance rotating x-ray tube anode assemblies having a large bore, thin-walled tubular stem. Most particularly, it relates to methods for joining a target, such as, for example, a molybdenum-alloy disk, to a stem, such as, for example, a niobium-alloy tubular stem portion, wherein inserts, such as, for example, tantalum-alloy inserts, are used as the bonding material to form a target/stem assembly and then connecting the target/stem assembly to a rotor body assembly.
X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. To release electrons, the filament in the tube is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the target portion of the anode, whereby they are abruptly slowed down. The anode, usually referred to as the target, is often of the rotating disc type so that the electron beam is constantly striking a different point on the target perimeter. The x-ray tube itself is made of glass and contains a cathode plate, an anode assembly including a rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the x-ray tube proximate to the rotor and overlapping therewith about two-thirds of the rotor length. The glass x-ray tube is enclosed in a protective casing having a window for the x-rays that are generated to escape the tube. The casing is filled with oil to absorb the heat produced by the x-rays. The casing in some x-ray tubes may include an expansion vessel, such as a bellows. High voltages for operating the tube are supplied by a transformer. The alternating current is rectified by means of rectifier tubes (or "valves") in some cases by means of barder-layered rectifiers.
For therapeutic purposes--e.g., the treatment of tumors, etc.--the x-rays employed are in some cases generated at much higher voltages (over 4,000,000 volts). Also, the rays emitted by radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles (for instance produced by a betatron), are used in radio therapy.
In producing new and improved anode assemblies for rotary anode x-ray application, it is not only necessary for the improved target/stem connection to survive a more rigorous environment in the x-ray tube but it must also be able to survive more rigorous manufacturing processes such as the vacuum firing at temperatures up to about 1500 degree(s) C. This requires that the target/stem connection provide the following benefits:
1. The bonding temperature be low enough to not adversely affect the properties of the TZM alloy which can lose significant strength at about 2000 degree(s) C. PA1 2. After bonding, have sufficient high temperature strength to support the bond through the additional manufacturing steps such as vacuum firing to about 1500 degree(s) C. PA1 3. The bond should be strong and should not be degraded by thermal excursions during normal x-ray tube operation up to 1500 degree(s) C. for extended periods of time. PA1 1) The strength and stiffness of the stem in bending and torsion are significantly improved over a small-diameter stem. 2) The tensile properties of the large-bore stem are nearly the same as a small-diameter stem. 3) The same thermal resistance offered by the small-diameter stem can be obtained by controlling the wall thickness of the large-bore stem.
For a particular set of bonding metals, such as in diffusion bonding, to work under the above parameters, it must have certain inherent properties. The first would be that the metals do not have a eutectic or peritectic reaction with the TZM target layer. Ultimately, the bond metals used, such as in diffusion bonding, should form only a simple binary solid solution with the molybdenum in the TZM alloy.
A basic rule in bonding dissimilar materials is that for a bond to occur there must be some intermixing of the elements between the two materials. Also, for this bond to have significant strength, the gradation of intermixing should approximate that of a binary diffusion couple where the two materials diffuse together in equal portions.
Conventional target/stem connections, to the extent they may be viable in conventional x-ray imaging systems, face a much more severe test in connection with the use of graphite members in x-ray tubes used in medical computerized axial tomography (C.A.T.) scanners. For the formation of images, a medical C.A.T. scanner typically requires an x-ray beam of about 2 to 8 seconds duration. Such exposure times are much longer than the fractions-of-a-second exposure times typical for conventional x-ray imaging systems. As a result of these increased exposure times, a much larger amount of heat (generated as a by-product of the process of x-ray generation in the target region) must be stored and eventually dissipated by the rotating anode.
X-ray tube performance can be affected by the balance of the anode assembly (a combination of a target/stem assembly and a rotor body assembly) which includes the target, the stem and the rotor. Specifically, during x-ray tube manufacturing, it is important to be able to balance the anode assembly and have it stay balanced during completion of the manufacturing cycle and during operation of the x-ray tube. As the size of x-ray tube targets has increased to six and one half inch (61/2") and beyond, it has proved difficult to maintain this balance and thus, reduced manufacturing yields and shortened x-ray tube operational lives have been experienced. Field evaluation of failed x-ray tubes has indicated that the imbalance of the anode assembly has often occurred in the region of the attachment of the target/stem or shaft assembly to the rotor body assembly.
Recently, the problem related to anode assembly failure due to imbalance reached a critical point. Due to the tremendous stresses undergone by the larger diameter x-ray tubes during continuous operation, the average tube life had been approximately 30,000 scan-seconds, utilizing the conventional threaded stem, Belleville washer mechanical connection.
Since approximately 20% of the failures were related to anode assembly imbalance, the need for new and improved methods of making an improved anode assembly having a new and improved, more durable target/stem assembly connection with the rotor body assembly that would eliminate the imbalance became apparent. The new and improved methods for making such a target/stem assembly desirably would provide sufficient anode assembly balance, when the target/stem assembly was connected to the rotor body assembly to form the anode assembly, during the operation life of the target/stem assembly-rotor body assembly connection while reducing significantly, if not entirely eliminating, tube failures due to anode assembly balance problems.